The protection of a three-phase electric motor against thermal overload is achieved in a known manner by three temperature monitor switches, wherein preferably these are bimetal switches. The temperature of each motor phase is sampled with its own temperature monitor so as to be able to detect even asymmetrical temperature increases, e.g. due to winding insulation damage. The three temperature monitor switches are connected in series. When the switching temperature of one of the temperature monitor switches is exceeded, it opens. The interruption is recognized in the evaluation electronics and the action required for the specific protection concept is triggered. Typically this means disconnection of the winding current flowing through the phase windings. Further, it is known hereby to provide a thermistor, by means of which phase-winding-dependent power regulation can occur. The resistance value of the thermistor, increasing with the temperature, is used to measure the current winding temperature, e.g. by measuring the voltage drop across the thermistor when a constant current is superimposed. When a temperature is too high, the power to the motor is reduced. Based on the assumption that with a properly functioning motor, temperatures in the phase windings are approximately equal, only one thermistor is used, in order to reduce the expense. In order to be able to evaluate the series circuit of the three temperature monitor switches by means of the evaluation electronics, two connections are needed to the signal electronics. Two connections to the electronics are likewise needed for evaluation of the thermistor using the evaluation electronics. Thus a total of four connections are needed between the electric motor and the evaluation electronics. This results in higher circuitry and component expense.
The invention has the basic object of reducing the circuitry expense for the thermal monitoring and power regulation of electric motors.